Currently, mobile communication systems require broad bandwidth and high linearity. Modern communications signals exhibit a high peak-to-average power ratio; therefore, an RF power amplifier is operated at a large power back-off to satisfy the high peak-to-average power ratio. Back-off corresponds to operating the power amplifier at an output power level below the peak power capability, usually done to accommodate a modulated signal with specified linearity requirements and a peak to average power ratio. However, typical RF power amplifiers with high linearity operating in a large back-off power domain exhibits low efficiencies.
For this reason, much effort has been put into increasing the efficiency of the RF power amplifier under back-off. In one approach, the Doherty power amplifier was introduced as a circuit that exhibits high efficiency at back-off. The Doherty amplifier achieves high efficiency at back-off through a main amplifier that operates into the high power added efficiency saturation region, and a peaking amplifier that supplies the signal peaks so that overall linearity can be restored. The Doherty configuration achieves load modulation by using the principle of “load pulling” using two devices. Thus in the Doherty power amplifier both the carrier and peaking amplifiers are connected in parallel across the load using a Doherty combiner.
A typical Doherty combiner comprises bulky, space-consuming distributed components, such as, micro-strip transmission lines or strip-line transmission lines, and is constructed on a low-loss substrate mounted on a large aluminum plate/block.
Alternatively, the Doherty combiner is also implemented as a surface mount component containing the space-consuming transmission lines. However, the presence of such bulky transmission lines impedes the integration of the power amplifier into small-sized and lower weight products; therefore, size issues associated with the Doherty amplifier design need to be addressed. The physical size of the transmission lines is proportional to the wavelength thus the physical size becomes particularly problematic as the frequency is decreased.
In addition, it is known that the cost of integrated circuits and modules (such as an RF amplifier) is usually proportional to the size of the integrated circuit die and/or the module; therefore, it is desirable to find techniques to reduce the increase in area because of the presence of the Doherty combiner. It is also highly desirable that the Doherty combiner circuit does not add any substantial cost or size to the integrated circuit (RF amplifier) containing the Doherty combiner circuits or to the development process, owing to re-designing, re-fabrication, and re-characterization of the different configurations (such as differing operating frequency, particularly low frequencies).
Therefore, it is desirable to find techniques that reduce the cost and size of the Doherty combiner and power amplifiers, and, at the same time, achieve better overall performance.